1. Field of the Invention
The present invention relates to a sheet handling apparatus for transporting sheets, and more particularly to an automatic document feeder for transporting original document sheets from a stacker to an exposure unit of an image forming apparatus such as a copying machine, a laser beam printer or the like.
2. Related Background Art
In conventional recycling automatic document feeders, the original sheet documents stacked on an original stacker are fed one by one, from the lowermost one, to the exposure unit of an image forming apparatus such as a copying machine, and are returned, after the required exposure operation, to the above-mentioned original stacker. Such feeding operation is repeated for a required number of cycles, one cycle being the feeding operations of a number corresponding to that of the original documents in the stack.
In such conventional automatic document feeders, speed control and stopping control are applied to the transportation of the original documents, in order to stop each document exactly on a platen glass.
However, such conventional feeders have required an unnecessarily long time for exchanging the original document because the stable control has only been achieved with loose timing, in order to absorb fluctuations in the sheet size, sheet quality and load of the system.
Also for controlling the timing of original document transportation, there has been provided at least one original detecting means, and, in a position requiring a high precision in the timing control, there has been employed a transmissive or reflective analog photosensor for detecting the document itself and generating an analog output signal.
However, in such conventional technology, in which the presence or absence of an original document has been identified by binary digitizing the output of said analog photosensor, difficulties such as deteriorated stopping accuracy of the original document, untidy stacking of the discharged original documents, and eventually sheet jamming, have been unavoidable due to the difference between the actual passing of the document and the electrical detection thereof resulting from fluctuations in the analog level or sensor response caused by dust, paper powder, temperature, humidity etc.